Method for planning subarea emission reduction strategies to improve ozone over a large area: A case of Taiwan

Abstract

Inappropriate reduction strategies can exacerbate ozone pollution. While previous research has primarily focused on developing emission reduction strategies for large areas, this study aimed to create a method for quickly planning emission reductions in subareas within a larger area. Using Taiwan (large area) as an example, this study illustrates how to determine the emission reductions required in each county (subarea) to achieve a 50% reduction in the number of DM8O3 (daily maximum 8-h average ozone) pollution station-days across the entire region (the preset goal). The methodology began with observational data to calculate the expected DM8O3 concentration improvements needed to meet the preset goal. CMAQ-DDM simulations were then conducted to evaluate cross-county O₃ transport, revealing the reciprocal impact of emissions between counties on air quality. The results indicated that NO_X emission reductions caused O₃ deterioration, leading to a focus exclusively on NMHC reductions. A system of simultaneous linear equations was constructed to link emission reductions, the cross-county O₃ transport, and the expected improvements. Linear programming was applied to calculate the required NMHC reductions for each county. The findings showed that most counties needed to reduce NMHC emissions by 30%–40%, while some required only 20%. CMAQ simulations of the improvements resulting from these emission reductions (referred to as validated improvements) confirmed that they met the expected improvements in most counties and across Taiwan as a whole. Finally, a nonlinear correction factor, defined as the ratio of validated to linear improvements, was introduced to refine the system of linear equations. This refinement effectively addressed the biases associated with using a system of linear equations to represent the nonlinear processes of ozone formation and ensured that all counties achieved their expected improvements.